A disproportionate number of new HIV infections occur in adolescent girls in sub-Saharan Africa (SSA). In parts of western Kenya, HIV prevalence rises from 1.3% in 13-14 year-olds to 12.8% by age 18 years. Bacterial vaginosis (BV), which doubles the risk of HIV acquisition and transmission, affects 20-50% of general population women in SSA and Kenya. For adolescent girls, the HIV/STI epidemic overlaps with broader reproductive health concerns. Menstrual hygiene management (MHM) is a pervasive problem across low- and middle-income countries and a lack of MHM materials negatively impacts girls' health and schooling. To attend school and obtain necessities such as sanitary products, soap and underwear, girls consequently often engage in exchange sex. To tackle these challenges, Co-Investigator Phillips-Howard conducted a cluster randomized controlled feasibility study following 644 girls aged 14-16 years old: after one year, menstrual cup use resulted in 35% lower (p=0.034) BV prevalence and 52% lower (p=0.039) STI prevalence compared to control condition of menstrual hygiene counseling. Based on our and others' research, we hypothesize menstrual cups protect against STIs by preserving or promoting a Lactobacillus-dominant vaginal microbiome (VMB). Numerous studies associated a Lactobacillus-dominant VMB with reduced risk of STI and HIV acquisition. We propose to study this proposed microbiome-related mechanism to understand how menstrual cup use leads to reduced BV and STIs, and the effect of menstrual cup use on evolution of the adolescent VMB. We will: Aim 1: Determine the influence of menstrual cup use on the VMB and how this is related to risk of BV and STIs; Aim 2: Identify the change in VMB as girls initiate sexual activity, and whether this is modified by menstrual cup use, and Aim 3: Augment trial pharmacovigilance through detection of E. coli on cups and correlation with VMB. The proposed study will be a longitudinal sub-study of 440 girls aged 14-16 years nested within a recently funded cluster randomized controlled trial to examine menstrual cup use versus cash transfer or control condition among 3,864 secondary school girls in Siaya County, Kenya (DfID/MRC/Wellcome Trust; PI: Phillips-Howard). To achieve our aims, we will characterize the VMB using high throughput amplicon sequencing of portions of the bacterial 16S rRNA gene. Participants (220 cup users, 220 controls) will be asked to provide a self-collected vaginal swab at baseline prior to allocation and at each semi-annual visit, for a maximum of 6 samples per subject (BL, 6-, 12-, 18-, 24-, 30-months). BV will be assessed at baseline at each semi-annual visit, and STIs (C. trachomatis, T. vaginalis, N. gonorrhoeae) at baseline and annually. HIV and HSV-2 are assessed at baseline and annually by the larger trial. Results from this study will provide understanding of whether microbiome modulating interventions being tested for adult women are applicable for adolescent girls. Menstrual cups could be a low-cost, multipurpose intervention to add to the prevention toolkit not just for girls, but for sex workers and women at high risk for HIV and STIs.